Need help diagnosing elevator stiffness

[rv8gibbo]

Would it be acceptable to machine down a few washers to achieve the desired shim?

[Thermos]

Originally Posted by Walt

Here's a picture I took many years ago of my HS center bearing right after I drilled the center bolt hole. There's certainly nothing "precision" about those spacers, I just worked them down with a hand file until they fit. The assembly probably still needs some more work, though; there's still a tiny bit of friction when the center bolt is tightened.

Dave

[JonJay]

Originally Posted by rvbuilder2002

Jon,
I don't think anyone has said it needs to be a precision bushing. It has been said that if you want to have your elevators operate properly, you need to accurately shim the gap that by design will exist between the elevator horn and the bearing. I am usually able to do this to a high enough level of precision with a combination of thick and thin washers, but it does occasionally require making a custom wash from aluminum to an specific thickness.
This may not have been detailed in the RV-6 plans/manual when you built your airplane, but I am pretty sure it has been in plans for RV's for many years now.

I am not aware of any data to imply that most of the fleet has an issue here.
Yes, a lot of RV's have more friction in the elevator circuit than is desirable for best handling qualities and stability. This bearing connection is not the only sorce of that friction (but it is one of the common ones). They seem to fly ok.... I think it is one of those things where people don't know, what they don't know.

I think choosing to not properly torque a fastener because we are pretty sure that the nut wont come off is a rather slippery slope to travel. Not that I disagree, but the reason we safety anything is for the "just in case" situations. As a standard practice we say that any bolt that could be subject to rotation must have a drilled bolt and cotter pin. This should not be considered any differently. Having said that, I do not think that is really a good choice for resolving this when simply installing the correct spacers will fix it the right way.

Say we decide it is ok. Then we must decide how big of a gap is ok. What if it is 3/16" on each side (not common, but not unheard of either)?

What it really comes down to is that if someone is going to be building and maintaining an airplane, it is smart to do it to the highest level practical. Properly doing an assembly process so that a bolt can be properly torqued falls within that standard.

Thanks Scott. I was just giving "food for thought" based on what was being said previously. I absolutely agree with you and Walt. I reworked mine some time ago after losing the argument with Walt (and I thought I was winning!, DARN!)
I was able to do it with a combination of washers.
I know I have told you this before, but thanks for being a valued resource on all things RV.
Carry on.

[Walt]

Originally Posted by Thermos

That's a picture I took many years ago of my HS center bearing right after I drilled the center bolt hole. There's certainly nothing "precision" about those spacers, I just worked them down with a hand file until they fit. The assembly probably still needs some more work, though; there's still a tiny bit of friction when the center bolt is tightened.

Dave

Well thanks for letting me "borrow" your picture, its the first good one I found doing a google search for images of the assy. Obviously I can't tell if you made the shims the correct size, but the "process" looks good

[BillL]

Good thread - I would like to add a little information. My right elevator had friction without the center. It floated with zero friction until the HS bolts were tight. It was clear that the steel brackets (or one of them) was causing an axial load on the heim bearings when tightened. So, the bolts were loosened and gap checked (measured) with a feeler gage. In this case, hand bending one bracket .015" reduced the friction by about 80%. Surely with more fiddling it could be minimized.

This is just to add to the possible measurements and adjustments future builders may consider to reduce friction.

[airguy]

I recently came across this as well - my plane came out of paint about three weeks ago and on the first long cross-countries I made I noticed the autopilot (Dynon) asking for a LOT of trim changes, first one way then the other. I hand flew for a while and played with the trim and determined I had either a sticky trim tab or a sticky elevator, was difficult to tell which while in flight but either way I chalked it up to paint overspray in a hinge somewhere.

After getting back home I first cleaned the trim tab hinge but it was moving freely already. The elevators were definitely stiff - relatively speaking. It was a small change but noticeable to me, and apparently to the autopilot too. I pulled the elevators and checked everything, and found that the "A&P" hired by my painter to disassemble and reassemble the airplane had removed the elevators and the washer spacers I had on the center bearing, and reinstalled without the spacers on the center bearing. The resulting sideloads and stiffness disappeared as soon as I reassembled it correctly.

I needed to pull the elevators anyway, to rebalance them after paint. Might be a minor detail ignored by many, but I did it anyway.

[vernon smith]

If someone with a lathe is not ready available to part off a spacer of the correct dimensions (use feeler gauges so your lathe friend only needs to make one set), brass and stainless shim stock is available from MSC or McMaster Carr in assorted packs for little money. They start at .005" and go up to .05 in .01 increments. Inexpensive punch sets are available from the same sources to match the through bold diameter. A simple office paper punch will work with the brass so long as you don't ask it to cut through more then .02. Standard AN washers as available in .031 thickness also.

These will come in handy in the future for lots of other situations. If you don't have a set of feeler gauges the shim packwill act as a rough one.

[Dbro172]

Originally Posted by airguy

I recently came across this as well - my plane came out of paint about three weeks ago and on the first long cross-countries I made I noticed the autopilot (Dynon) asking for a LOT of trim changes, first one way then the other. I hand flew for a while and played with the trim and determined I had either a sticky trim tab or a sticky elevator, was difficult to tell which while in flight but either way I chalked it up to paint overspray in a hinge somewhere.

After getting back home I first cleaned the trim tab hinge but it was moving freely already. The elevators were definitely stiff - relatively speaking. It was a small change but noticeable to me, and apparently to the autopilot too. I pulled the elevators and checked everything, and found that the "A&P" hired by my painter to disassemble and reassemble the airplane had removed the elevators and the washer spacers I had on the center bearing, and reinstalled without the spacers on the center bearing. The resulting sideloads and stiffness disappeared as soon as I reassembled it correctly.

I needed to pull the elevators anyway, to rebalance them after paint. Might be a minor detail ignored by many, but I did it anyway.

Exact same here. I'm glad i came across this thread. I had stiff elevator after paint and similar auto pilot issues. I loosened this bolt a tad and issue solved. Now I think I will look at this again; install a thin washer and torque to spec and hopefully be back to normal.

[dbaflyer]

I just did this for my RV9A. It only took one AN960-416L washer on the right side. This was in addition to the metal washers(don't have the part number, but they are silver in color) the plans call out to install on either side of the bearing.

[Wunderon]

been fiddling with this for a few days. at the moment it appears my elevator horns are not quite squarely welded to their tubes. so torquing the 1/4 bolt causes an axial load outboard. I think I can resolve this by using slightly oversized spacers to offset the flex.